1. Field of the Invention
The present invention relates to a radar apparatus that is installed inside a vehicle or the like, for example, and is constructed so as to be capable of detecting a distance to a measured object such as an obstacle.
2. Description of the Related Art
A radar apparatus disclosed in Japanese Laid-Open Patent Publication No. H07-244154 is one example of this kind of radar apparatus. As shown in FIG. 1 of the publication, the radar apparatus includes a trigger signal generating circuit, a transmitting means, a reflected signal receiving circuit, and a distance detecting circuit. Here, the reflected signal receiving circuit includes a PD (photodiode), a logarithmic amplifier circuit, an A/D converter, and a reflected waveform memory. In this radar apparatus, the trigger signal generating circuit supplies a transmission start signal to the transmitting means and a conversion start signal to the A/D converter. Next, the A/D converter starts A/D conversion from a point when the start signal is inputted from the trigger signal generating circuit. By doing so, the waveform of a reflected signal from a start of transmission by the transmitting means is stored at sampling intervals in temporal order as digital data in the reflected waveform memory. On the other hand, the distance detecting circuit detects rises in the waveform of the reflected signal stored in the reflected waveform memory and, based on the product of the number of pieces of digital data that have been stored and the sampling interval, finds the time from the start of recording for the waveform of the reflected signal (i.e., the start for A/D conversion by the A/D converter) until the detected rise, and detects the distance to the reflecting body from the calculated time and the speed of light.
By investigating the radar apparatus described above, the present inventors discovered the following problems. In the above radar apparatus, circuits such as the trigger signal generating circuit and the reflected signal receiving circuit are normally designed to operate in synchronization with an internal clock outputted by a crystal oscillator disposed inside the apparatus. This means that in the above radar apparatus, the timing of the start of recording of the waveform of the reflected signal by the A/D converter that starts A/D conversion based on the start signal and the sampling clock of the A/D converter are also thought to be synchronized with the internal clock. On the other hand, since the reflected signal is proportional to the distance to the reflecting object, the rising point of the waveform of the reflected signal is asynchronous with the sampling clock. For this reason, at such asynchronous part, a measurement error of up to one cycle of the sampling clock is always present. Since the resolution for the time from the start of recording of the waveform of the reflected signal to the rise is around one cycle of the sampling clock, the resolution for the distance to the reflecting object is the product of one cycle of the sampling clock and the speed of light, resulting in the problem that high resolution measurement is not possible. There is a further problem in that since the measurement error described above is unrelated to the time from the start of recording for the waveform of the reflected signal to the detected rise, the shorter the time from the start of recording for the waveforms of the reflected signal until the detected rise, that is, the shorter the distance to the reflecting object, the larger the relative error in the measurement of distance.
The above problem can be improved by increasing the frequency of the sampling clock. However, as the frequency of the sampling clock is increased, it normally becomes necessary to use high-speed electronic components, which are costly compared to low-speed electronic components, resulting in the problem of a large increase in the manufacturing cost of the radar apparatus.
The present invention was conceived in view of the above problems, and it is a principal object of the present invention to provide a radar apparatus that has increased resolution for measuring distances without increasing the frequency of the internal clock.